Deep mantle dynamics (2)

Origin and thermal evolution of the deep mantle - magma ocean, relationship with current structures.

This lecture was devoted to the geodynamic considerations that allow zones such as ULVZs to be maintained at the base of the mantle. In particular, the questions asked are:

• Is mantle convection vigorous enough to dynamically maintain small ULVZs denser than the surrounding mantle without forming a dense uniform layer at the base of the mantle?
• How can ULVZs be prevented from being drawn into the surrounding mantle?
• What is the effect of accumulated subduction plates in the D" on the shape of LLSVPs and the geographical distribution of ULVZs?

Geodynamicists are proposing high-resolution 2D convection models (on the order of kilometers at CMB), with three different compositions (LLSVP, ULVZ, ambient mantle). ULVZs are initially introduced as a homogeneous layer on the CMB. They numerically solve the conservation of mass, momentum and energy equations in the Boussinesq approximation (incompressible mantle), with temperature-dependent viscosity, and introduce tracers to locate the compositional field. They show that if the density contrast between ULVZs and ambient mantle is at least 5%, ULVZs can form at the base of the mantle and remain at the edge of LLSVPs. These 2D calculations have recently been extended to 3D. These models are also able to reproduce the steepness of LLSVP edges.

Finally, we raised the question of the possible presence, early in the Earth's history, of a magma ocean, a stable layer of dense fluid followed by slow fractional crystallization, which could have formed domes of particular composition corresponding to the LLSVPs observed seismically today.